1. Field of the Invention
The present invention generally relates to the detection of gamma rays, and more specifically to a semiconductor P-type/Intrinsic/N-type (P-I-N) gamma ray detector and fabrication method which provide potential barriers for blocking leakage current and low resistance ohmic contacts.
2. Description of the Related Art
Gamma ray detectors are used in numerous fields including nuclear instrumentation, medical imaging, biological research and dosimetry. An especially useful application of a semiconductor gamma ray detector is as a sensing element in a gamma-ray spectrometer.
Cadmium telluride (CdTe) is an effective solid-state compound semiconductor material for detecting gamma rays, since it has a relatively high atomic number of 50 which provides a large cross section for ray interaction. It also has a large bandgap of 1.5 eV which enables operation at room temperature. The addition of zinc (Zn) to cadmium telluride produces cadmium zinc telluride (CdZnTe) which has a higher bandgap of approximately 1.6 eV.
A basic P-I-N semiconductor gamma ray detector includes a wafer of intrinsic cadmium telluride or cadmium zinc telluride with doped contacts formed on the opposite surfaces thereof. A reverse biasing electric field is applied across the contacts. Gamma rays passing through the wafer liberate electron-hole pairs which are swept to the respective contacts by the electric field and generate electrical pulses in an associated electronic unit.
A detector of this type is described in an article entitled "LARGE, HIGH RESOLUTION CdTe GAMMA RAY SENSORS", by T. Hazlett et al, in IEEE Transactions on Nuclear Science, Vol. 33, No. 1, Feb. 1986, pp. 332-335. A P-type contact is formed on a surface of a cadmium telluride wafer by diffusion of gold (Au), and an N-type contact is formed on the opposite surface of the wafer by diffusion of indium (In).
A disadvantage of this structure is that the wide bandgap which makes cadmium telluride effective as a gamma ray detecting material also makes it difficult to form direct metal contacts in a controllable manner thereon due to interface states between the wafer and the metal contacts.
The basic detector is also subject to substantial leakage currents caused by injection of minority carriers from the contacts into the wafer. These leakage currents create undesirable effects including electrical noise and a reduction of the voltage which can be applied across the detector, thereby degrading the charge collection efficiency and energy resolution in the gamma ray energy spectrum. This is especially detrimental when the detector is used as a sensing element for gamma ray spectrometry.
An improved P-I-N gamma ray detector is described in an article entitled "GAMMA RAY DETECTORS WITH HgCdTe CONTACT LAYERS", by F. Ryan et al, in Applied Physics Letters, Vol. 46, No. 3, Feb. 1985, pp. 274-276. P- and N- doped layers of mercury cadmium telluride (HgCdTe) of composition (Hg.sub.1-x Cd.sub.x Te) are formed on the opposite surfaces of a cadmium telluride wafer. The values of x for the P- and N-doped layers are 0.6 and 0.31 respectively. Ohmic contact layers of gold and indium are formed on the P- and N-doped layers respectively.
The mercury cadmium telluride layers separate the metal contacts from the cadmium telluride wafer, thereby eliminating the metal/CdTe interface states and associated fabrication problems. The mercury cadmium telluride layers may also produce potential barriers on the order of 0.6-0.7 eV which would suppress the flow of leakage currents from the ohmic contacts into the wafer, although the possibility of such barriers is not acknowledged in Ryan's article.